In general, blood pressure in an artery periodically increases and decreases depending on systole and diastole of the heart. In the periodic change of the blood pressure, the maximum of the blood pressure is referred to as systolic pressure PSYS, and the minimum of the blood pressure is referred to as diastolic pressure PDIAS. Typically, the blood pressure is indicated by PSYS/PDIAS, and its measurement unit is mmHg.
A method of measuring blood pressure employs the principle of non-invasive blood pressure measurement. In this principle, a part of an arm is pressed by an oppressing pressure. At this time, the oppressing pressure is indicated by PC, and the blood pressure in arteries distributed in the arm is shown by BP. The principle is such that as tissue around the arteries transfers the oppressing pressure PC into the blood pressure BP, the arteries are closed or opened according to the oppressing pressure PC and the blood pressure BP.
If the oppressing pressure PC is greater than the blood pressure BP, the arteries are closed. To the contrary, if the oppressing pressure PC is less than the blood pressure BP, the arteries remain opened. Hence, if the oppressing pressure PC sufficiently increases, i.e. PC>Psys, all the arteries are closed. On the other hand, when the oppressing pressure PC on the arm decreases, the arteries, which have been closed, are somewhat opened, and thereby it is possible at a position on the arm to ausculate heart rate and blood vortex flowing through the narrow arteries. This is called the Korotkoff Sound.
After hearing the Korotkoff Sound, the oppressed pressure further decreases to allow the arteries to fully open. That is, if the oppressing pressure PC becomes less than the diastolic pressure PDIAS, the Korotkoff Sound disappears.
According to the principle of the non-invasive blood pressure measurement, the oppressing pressure PC at a point of a time of hearing the Korotkoff Sound is recorded as the systolic pressure PSYS, and the oppressing pressure PC at a point of a time when the Korotkoff Sound disappears is recorded as the diastolic pressure PDIAS.
A mercury sphygmomanometer is one device that uses the principle of the non-invasive blood pressure measurement as described above. The conventional mercury sphygmomanometer includes a blood pressure cuff which is wound on a users arm and filled with air to generate pressure enabling the cuff to oppress the users arm, a mercury column for indicating the pressure PC in the cuff (it indicates the pressure mmHg by its height), a bulb shaped air injector for injecting air into the cuff with a users hand so as to increase the pressure in the cuff, and a decompression valve for controlling a decompression rate by releasing a screw with the users hand.
As shown in FIG. 1 in the mercury sphygmomanometer, the air injector 10 has a rubber-made ball shape with one side connected to the cuff 20 and the other side communicated with atmosphere A. The air injector 10 has one-way valves 11 and 11′ mounted on a side connected to the cuff 20 and the other side communicated with the atmosphere A, respectively. A decompression valve 30 is installed near the one-way valve 11 connected to the cuff 200. Therefore, when the decompression valve 30 is fully closed and the air injector 10 is pressed in a direction marked by an arrow M in FIG. 1, the inner pressure in the air injector 10 increases. Thus, the inner pressure of the air injector 10 causes the atmosphere-sided one-way valve 11′ to be closed and the cuff-sided one-way valve 11 to be opened, so that the air in the air injector 10 is injected into the cuff 20 in a direction marked arrows in FIG. 1. When the pressure applied to the air injector 10 is released, the air injector is relaxed by its elasticity. In other words, the air injector 10 expands outward in a direction marked by the arrow M to generate negative pressure therein, so that the cuff-sided one-way valve 11 is closed and the atmosphere-sided one-way valve 11′ is opened. Hence, the exterior air is introduced into the air injector 10. These processes repeat and generate high pressure in the cuff 20.
When the decompression valve 30 is opened to decrease the pressure in the cuff 20 in order to measure the user blood pressure, the air is discharged from the cuff 20 so that the pressure drops in the cuff 20. However, since the decompression valve 30 has a configuration in the form of a screw, there is required skill in controlling a decompression rate using the decompression valve because of the sensitivity of the decompression valve.
It a user intends to measure his/her blood pressure using a mercury sphygmomanometer, the cuff 20 is wound on his/her arm and then the decompression valve 30 is completely closed. Next, when air is injected into the cuff 20 through the air injector 10 so that the pressure PC in the cuff 20 is greatly higher than the systolic pressure PSYS, as described above, the pressure of the mercury sphygmomanometer 40 connected to the cuff 20 increases. After the air is introduced into the cuff 20 so that the oppressing pressure PC increases to be sufficiently higher than the systolic pressure PSYS, the decompression valve 30 is somewhat opened to slowly decompress the oppressing pressure PC at a rate of −3˜5 mmHg/sec which is a recommended decompression rate. At this time, the user instantly reads the height of the mercury column 40 at a time point of hearing the Korotkoff Sound through a stethoscope, so as to determine the systolic pressure PSYS, and continues to decompress the oppressing pressure PC. Further, the user reads the height of the mercury column at a time point when the Korotkoff Sound completely disappears and determines the diastolic pressure PDIAS.
However, there is required sufficient skill in measuring the blood pressure using the conventional mercury sphygmomanometer because the user has to determine the time points when the Korotkoff Sound occurs and disappears by listening for the Korotkoff Sound with his/her ears using the stethoscope, instantly read the height of the mercury column at those time points with his/her eyes, and control the decompression rate to be −3˜-5 mmHg/sec with his/her hand.
On the other hand, in the case of not the mercury sphygmomanometer but an automatic sphygmomanometer, a computer automatically controls a small air pump or valve to maintain a constant decompression rate However, this requires very complicated and accurate control algorithms.
Therefore, if the regulation of the decompression valve is completed, motive power for air outflow increases with the pressure PC in the cuff 20, as seen in the graph of FIG. 3. As a result, air outflow velocity increases while the decompression rate also increases. On the other hand, when the pressure PC decreases, the motive power for the air flow is reduced, resulting in decreasing the decompression rate. In other words, the pressure PC decreases not linearly but exponentially. FIG. 3 is a graph illustrating a decompression rate in proportion with a time in the conventional mercury sphygmomanometer under a condition of a constant pressure PC according to the opening and closing of the decompression valve. It is understood that the decompression rate is generally changed according to the opening and closing of the decompression valve, but the pressure PC is exponentially decompressed.
The operation of the decompression valve 30 shown in FIG. 1 is described as follows:
As the air is discharged from the cuff 20 through the decompression valve 30 by the pressure PC which is higher than the atmospheric pressure, the pressure PC drops. The decompression rate u of the pressure PC, i.e. variation of the pressure PC per unit time (dPC/dt) is in proportion to the air outflow Q, and can be expressed by a following equation;u≡dPC/dt∝Q, wherein since the air outflow Q can decrease as the fluid resistance R of the decompression valve 30 increases, the air outflow Q is in inverse proportion to the fluid resistance R. This can be expressed by a flowing equation:u≡dPC/dt∝Q∝1/R 
Therefore, the decompression rate is in inverse proportion to the fluid resistance. This can be expressed by a following equation:
            ⅆ              P        C                    ⅆ      t        ∝      1    R  
Meanwhile, when the pressure PC which is a motive power for the air outflow, increases, the decompression rate is in proportion to the pressure PC. The decompression rate of the pressure PC is in proportion to the pressure PC, and in inverse proportion to the fluid resistance R, as expressed by a following equation:
            ⅆ              P        C                    ⅆ      t        ∝            P      C        R  
Here, if the fluid resistance R is constant, i.e. the extent of opening and closing of the decompression valve can be constantly maintained, the decompression rate dPC/dt is in proportion to only PC. Therefore, when the pressure PC increases, the decompression rate also accelerates. Further, when the pressure PC drops, the decompression rate drops. Thus, the variation of the pressure PC according to time is in the form of exponential function, as shown in FIG. 3.
As described above, if the proper decompression rate of about −3˜-5 mmHg/sec required to measure the blood pressure is maintained regardless of the pressure PC, the fluid resistance R must be in proportion to the pressure PC. That is, if the fluid resistance R is changed in proportion to the pressure PC, the equation can be expressed as follows:
      R    =                  P        C            k        ,wherein k is constant.
If R in the foregoing equation is substituted with this equation, a following equation can be obtained:
            ⅆ              P        C                    ⅆ      t        ∝            P      C                      P        C            k      wherein k is constant. Thus, the decompression rate becomes constant, and the pressure PC linearly decreases.
Based on the above-mentioned principle, the applicant has filed a Korean Patent Application Serial No. 2005-0042317, entitled with “Pressure-Linked Automatic Decompression Valve for Sphygmomanometer”, in which the decompression valve makes a fluid resistance R to be in proportion to pressure PC using the elasticity of a spring so as to linearly decompresses the pressure. As shown in FIG. 4A, according to the Korean patent application, the pressure-linked automatic decompression valve 100 is used in an upright position, and has an outer cylinder 112 and a piston shaft 115. The decompression valve 100 has an overlapping region defined by the movement of the piston shaft 115 in the outer cylinder 112 and having a predetermined length L. Further, the decompression valve 100 is connected at a lower portion thereof to the connector of the sphygmomanometer so that the pressure PC of the cuff is applied to a piston 114.
In the Korean Patent Application, when air is injected into the cuff, the pressure PC increases to push the piston 114. Thus, the piston shaft 115 moves upward to maximize the overlapping length L. At this time, a compression spring 116 interposed between the piston 114 and an inner cylinder is compressed to the maximum extent. When the air injection is interrupted, the air is discharged from the cuff through a gap between the outer cylinder 112 and the piston 114, and the overlapping region with the predetermined length L. At this time, the gap between the piston 114 and the outer cylinder 112 is sufficiently enlarged while the thickness of the piston is made narrow, so that the air can pass by the piston 114 without being subject to resistance. However, a gap between the piston shaft 115 and the outer cylinder is made very narrow to cause fluid resistance. Since the width of the gap is constant, the fluid resistance is in proportion to the overlapping length L. In other words, if the overlapping length L increases, the fluid resistance also increased to prevent the air outflow. As the pressure PC increases at an initial time when the air outflow starts, a motive power for the air outflow grows. The air outflow rate to the constant fluid resistance is rapid. However, since the overlapping length L also extends, the fluid resistance increases to reduce the air outflow rate. The pressure PC and the overlapping length L operate in opposite directions.
In addition, when the pressure PC drops as the air outflow proceeds, force to compress the spring 116 is reduced. As a result, the spring 116 returns to an initial position, and thereby the overlapping length L further becomes shorter. The decreasing of the pressure PC means that the motive power for the air outflow is reduced. Meanwhile, the reduction of the overlapping length L means that the fluid resistance decreases. Even though the motive power decreases to decelerate the air outflow rate, the fluid resistance is lowered along with the deceleration of the air outflow rate, thereby offsetting the air outflow rate caused by the reduction of the motive power. That is, as the fluid resistance R is reduced in proportion to the reduction of the pressure PC which is the motive power for the air outflow, it is possible to obtain the constant air outflow rate regardless of the pressure PC. Thus, the pressure PC can be linearly decompressed at a constant rate.
In the decompressing process as described above, predetermined friction Ff is generated during the movement of the piston 114 and the piston shaft 115, and the compression and extension of the spring. When the instrument is in an upright position, the weight Wn of the piston 114 and the piston shaft 115 can offset an upward friction caused by the downward movement of the piston 115 and the piston shaft 114, so that the decompression valve can perform an ideal operation.
The pressure-linked automatic decompression valve 100 as described above operates based on the principle in which the compressed length of the spring 116, which is the overlapping length L, extends to be in proportion to the pressure PC and the fluid resistance R of the valve.
However, the spring 116 tends to be compressed and deviated in not only a vertical direction but also a horizontal direction (see FIG. 4B). Therefore, the spring unnecessarily frictionizes the outer cylinder 112 as shown in FIGS. 4A and 4B.
As shown in FIG. 4B in which a part of the spring 116 of FIG. 4a is shown, since the spring 116 is subjected to a vertical pressure P upward and downward, the spring 116 tries to horizontally deviate across the criteria point S. As the horizontal deviation of the spring 116 is irregularly generated, the friction between the spring 116 and an inner surface of the outer cylinder 112 irregularly occurs. Thus, it is impossible to offset the friction by using the weight of the valve. This causes the valve 100 not to operate smoothly. Further, the valve 100 according to the Patent Application must be used in an upright position. However, in some cases, the valve 100 cannot be positioned uprightly under the circumstance of measuring the blood pressure. Therefore, there is required a solution in which the valve 100 is maintained to be upright regardless of the slope of a floor on which the valve is located.